1. Technical Field of the Invention
The present invention relates to spread spectrum communications and, in particular, to signal processing operations for a rake receiver and an integrated circuit chip for performing those operations.
2. Description of Related Art
Many multiple access techniques are known in the art. For example, wireless communications between many users currently use, for example, time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) techniques that allow multiple users to share a limited communications frequency spectrum.
Considerable attention is currently being directed to CDMA techniques which are viewed by those skilled in the art as possessing numerous advantages over other multiple access techniques. These advantages stem from the wideband nature of the CDMA communication format which, contrary to narrowband TDMA and FDMA techniques, spreads signal energy over a wide bandwidth. By spreading in this manner, the frequency spectrum can be reused more effectively to increase communications capacity. Additionally, CDMA systems operate without substantial adverse affect due to the common problems of multipath and fading that plague the wireless terrestrial communications channel. In fact, CDMA systems take advantage of the multipath phenomenon by using diversity combiners to capture the communications energy of multiple paths and thus enhance communications performance.
In order to demodulate two distinct paths of a communications channel, the time offset between those two paths must be determined. Using a demodulator, the CDMA receiver searches through a sequence of offsets and measures the corresponding received energy. If the measured energy exceeds a threshold, the receiver assigns a demodulation element (also referred to in the art as a “finger” of the rake receiver) to the signal representing that particular offset. Once all the paths for a channel are determined and assigned fingers, the signals on those paths are demodulated by the respective fingers and the symbols output from each finger are summed together to recover the communicated signal. Given the time varying nature of the wireless terrestrial communications channel, it is possible, and in fact likely, that the relative offsets between the paths will vary, and thus the individual fingers of the rake receiver must possess the ability to track, adjust and account for offset time changes.
The functioning of the fingers of the rake receiver is asynchronous with respect to each other due to the asynchronous nature of the plural propagation paths. Since the combination operation requires the availability of synchronous symbols from the plural paths of a single channel, a buffer is typically employed to temporarily store the symbol data output from each finger. When the last symbol to be combined for a given channel is output from its processing finger (i.e., the finger tracking the most delayed propagation path), the combining operation is performed with respect to the buffer stored symbols. Locations within the buffer are then made free to store subsequently arriving symbols, and the process repeats. This operation for collecting (accumulating) related symbols carried over different paths for combining is referred to in the art as “de-skewing.”
There are a number of recognized drawbacks with the conventional, prior art, buffer-based de-skewing operation. The computations necessary for demodulation are performed locally, that is, within the fingers of the rake receiver. Because of this, it is very difficult, if not impossible, to obtain computation reductions due to the redundant processing operations being performed in each of the fingers. Additionally, each finger submits sequence generation requests in an asynchronous way. Thus, the different fingers will reiterate requests to receive the same sequences at different times. Nonetheless, the sequences used by the fingers are the same, and thus the receiver inefficiently operates to generate more than one sequence per sequence period. All of these issues lead to receivers that are complex in implementation and power consuming in operation.
There is accordingly a need for an improved rake receiver that eliminates the need for separate, local performance of demodulation calculations in each finger.